FIG. 12A shows a sectional view of an oxygen sensor for determining the oxygen concentration of a measurement gas. A pair of electrodes 132 and 134 formed from, for example, porous platinum and having the form of a thick or thin film are formed on opposite sides of an oxygen-ion conductive solid electrolyte substrate 122. A voltage is applied between the electrodes 132 and 134 and a resultant current is measured in order to determine an oxygen concentration. According to the configuration of FIG. 12A, a housing 124 having a small oxygen diffusion hole 234C formed therein covers the negative electrode 134 so as to limit diffusion of oxygen through the oxygen diffusion hole 234C, so that a value of current proportional to oxygen concentration is obtained. Such an oxygen sensor must be provided with a heater for heating a solid electrolyte substrate to a temperature of about 400° C. to 900° C. in order to activate the solid electrolyte substrate. However, since the opposite sides of the solid electrolyte substrate bear respective electrodes, attachment of the heater to the oxygen sensor is difficult.
To solve the above problem, a flat limiting-current-type sensor as shown in FIG. 12B is used. As shown in FIG. 12B, a negative electrode 134 and a positive electrode 132 are disposed on the same side of the solid electrolyte substrate 122. Since the electrodes 132 and 134 are disposed on the same side of the solid electrolyte substrate 122, the flat limiting-current-type sensor has an advantage in that a heater can be readily disposed on the other side of the solid electrolyte substrate 122. The configuration of the flat limiting-current-type sensor is described in detail in Japanese Patent Application Laid-open (kokai) No. 2-147853 which corresponds to U.S. Pat. No. 5,348,630 filed by the present applicants.
FIG. 12C shows the flat limiting-current-type sensor of FIG. 12B as viewed in the direction of arrow C of FIG. 12B, i.e., FIG. 12C shows a side view of the flat limiting-current-type sensor. Since the negative electrode 134 and the positive electrode 132 are disposed on the same side of the solid electrolyte substrate 122, the area of the negative electrode 134 (positive electrode 132) is subsequently half that in the case of the limiting-current-type sensor of FIG. 12A. Accordingly, the flat limiting-current-type sensor of FIG. 12B has a problem in that an element resistance becomes higher and that measurement accuracy becomes poorer, as compared to the sensor of FIG. 12A.
In view of the foregoing, an object of the present invention is to provide a flat limiting-current-type sensor having an improved measurement accuracy for a given device size.
The present inventors realized that, in a sensor element, a negative electrode and a positive electrode might not operate in a similar manner. The flat limiting-current-type sensor repeats an oxygen-related pumping cycle. Specifically, oxygen is pumped into a solid electrolyte substrate in the form of ions at the interface between the solid electrolyte substrate and the porous negative electrode. The pumped-in oxygen ions are transmitted through the solid electrolyte substrate. Then, the transmitted oxygen ions are pumped out in the form of oxygen at the interface between the solid electrolyte substrate and the porous positive electrode. The present inventors assumed that there might be a difference between the readiness of reaction for pumping in oxygen in the form of ions and the readiness of reaction for pumping out oxygen ions in the form of oxygen. Specifically, according to assumption of the inventors, in a conventional flat limiting-current-type sensor as shown in FIG. 12C, the area of the negative electrode is equal to that of the positive electrode; consequently, transmission of ions is controlled by the negative electrode or the positive electrode, whichever is lower in terms of readiness for reaction, with a resultant increase in element resistance. The inventors conducted experiment on the basis of the assumption and obtained an appropriate area ratio between the negative electrode and the positive electrode.